Variation of Correlative Cycle Sequences Across a Platform-to-Foreland Basin Transition, Lower Mississippian, Southwestern Montana and East-Central Idaho

S. K. Reid, S. L. Dorobek

The Lower Mississippian Mission Canyon formation and stratigraphic equivalents in southwestern Montana and east-central Idaho consist of cyclic sequences of platform limestones/dolomites that grade westward into cyclic slope and basinal facies. Deeper water carbonate rocks were deposited on a ramplike profile that graded downslope into mixed carbonate-siliciclastic starved basin facies of the Antler foreland basin.

Third-order platform cycles consist of 20 to 50-m thick shallowing-upward peritidal carbonates. Superimposed fourth- and fifth-order cycles (< 1-5 m thick) also are recognizable at the tops of third-order cycles.

Third-order cyclic sequences (55-140 m thick) from slope environments consist of laminated, cherty limestones which become progressively more bioturbated and skeletal in the shallowest parts of the sequences.

Basinward, third-order cycles (75-150 m thick) consist of basal plane-laminated, pyritic, calcareous siltstones which grade upward into bioturbated, silty, skeletal wackestones/packstones with common whole fossils in upper parts of the cycles. Superimposed fourth and fifth-order cyclicity (2-4 m thick) is expressed as variations in ichnofauna or degree of bioturbation.

Platform cycles are asymmetrical, but deeper water cycles are more commonly symmetrical. Basal facies in deep-water cycles disappear in stratigraphically higher parts of the section, possibly due to shallowing of the Antler foreland basin through platform progradation and eustatic sea level lowering in the late Early Mississippian.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.